1. Field of the Invention
The present invention generally relates to a wireless communication technical field. More particularly, the present invention relates to a communication node, a wireless communicant system and a data relay method that use the multi-hop scheme, the relay scheme and the MIMO (Multi-Input Multi-Output) scheme.
2. Description of the Related Art
In a conventional relay method, signal transmission and signal receiving are performed in a time division manner such that a transmit signal and a receive signal does not interfere with each other at the relay node. However, communication capacity is largely reduced due to the time division scheme so that the communication capacity of the relay system is limited.
The conventional relay method is described with reference to FIG. 1. The reason why the communication capacity is largely limited is described in the following.
In FIG. 1, following parameters are defined.
s: a transmit signal vector sent from a source node
r: a receive signal vector
K: a number of usable relay nodes
Hk: a k-th channel between source node and relay node (backward channel), 1≦k≦K
Gk: a k-th channel between relay node and receive node (forward channel), 1≦k≦K
Wk: weight matrix in the k-th relay node, 1≦k≦K
Ek: power limit coefficient in the k-th relay node 1≦k≦K (for limiting maximum power in each relay node)
nk: noise component in the k-th relay node
z: noise component of receive node
E(·): ensemble mean value of variable
As shown in FIG. 1, a receive signal that is sent from a source node 1 using plural antennas and that is received by each relay node 2 can be represented as an equation (1).yk=Hks+nk  (1)The relay node 2 multiplies this signal by the power limit coefficient Ek and the weight matrix Wk to produce a relay transmit signal as shown in the following equation (2).Xk=EkWkyk  (2)
As a result, a receive signal at a destination node 3 can be represented as the following equation (3).
                    r        =                                            ∑                              k                =                1                            K                        ⁢                                          G                k                            ⁢                              x                k                                              +          z                                    (        3        )            
When transmission and receiving are performed simultaneously in each relay node, a signal sent from a relay node is received by another relay node so that interference occurs. As a result, the relay node cannot properly receive a signal sent from the source node 1.
Therefore, in the conventional relay method, as shown in FIG. 1, a time slot in which the source node 1 sends a signal and the relay node receives the signal and a time slot in which the relay node sends a signal and the destination node 3 receives the signal are separately provided. Thus, two time sols are necessary for information sent from the source node 1 to arrive at the destination node 3. Communication capacity of this case is represented as the following equation (4).Cupper=E(Hk,Gk){(½)I(s;y1 . . . ,yK, r|X1, . . . , XK)}  (4)The term of “½” in the equation (4) is caused by the fact that the two time slots are used. Therefore, the communication capacity is limited as a whole.
By the way, there are following documents relating to the technical field of the present invention.
Non patent document 1: Rohit U. Nabar, et al., “Capacity Scaling Laws in MIMO Wireless networks”, Allerton Conference on Communication, Control, and Computing, Monticello, Ill. pp. 378-389, October 2003.
Non patent document 2: Hui Shi, et al., “A Relaying Scheme using QR Decomposion with Phase Control for MIMO Wireless Networks,” IEEE International Conference on Communications, Volume 4, 16-20 May 2005 Page(s): 2705-2711.
Non patent document 3: G. D. Golden, G. J. Foschini, R. A. Valenzuela, and P. W. Wolniansky, “Detection Algorithm and Initial Laboratory Results using the V-Blast Space-Time Communication Architecture”, Electronic Letters, Vol. 35, No. 1, Jan. 7, 1999.
The above-mentioned conventional technique has a following problem.
As mentioned above, in the conventional relay system, the time slot in which the source node 1 sends a signal and the relay node receives the signal, and the time slot in which the relay node sends a signal and the destination node 3 receives the signal are separately used. By separately providing the time for the relay node to send a signal and the time for the relay node to receive a signal, interference among plural antennas in a relay node and interference among relay nodes can be avoided. However, as represented by the multiplication of “½” in the equation (4) representing the communication capacity, there is a problem in that the communication capacity is reduced.